1. Field of the Invention
This invention relates to a focus detecting device for detecting the focus state of an objective lens by sensing the light from a light source reflected by an object by means of a sensor provided near a plane conjugate with respect to the predetermined imaging plane of the objective lens, and particularly to a so-called TTL-active type focus detecting device applicable to a small optical apparatus such as a camera.
2. Description of the Prior Art
Focus detecting devices of this type have already been shown, for example, in U.S. Pat. No. 4,357,085 and Japanese Laid-open Patent Application No. 22210/1982 (laid open on Feb. 5, 1982). In such devices according to the prior art, however, a light-receiving element having the center of its light-receiving surface disposed at a position whereat the image of a light source is formed during the in-focus of the objective lens is disposed along a plane conjugate with the predetermined imaging plane of the objective lens (the object surface) and therefore, the light-receiving efficiency of the light-receiving element during non-in-focus has been low and the detection accuracy in this case has not been sufficient.
FIGS. 1 and 2 of the accompanying drawings show the typical optical system of the TTL-active type focus detecting device. In FIG. 1, a light beam emitted from a light source 1 passes through a projection lens 2, whereafter it is reflected by mirrors 3 and 4 and is projected onto an object, not shown, through an objective lens 5. The light from the light source 1 reflected by the object passes through the objective lens 5, whereafter it is reflected by the mirrors 4 and 3 and is imaged on the light-receiving surface of a light-receiving element 7 through a light-receiving lens 6. In FIG. 2, F designates the image pick-up surface (the predetermined imaging plane) of a camera and, when this device is incorporated into the camera, the light-emitting surface of the light source 1 and the light-receiving surface of the light-receiving element 7 are disposed in a plane conjugate with the surface F. In this case, the light from the light source 1 reflected by the object is converged as shown in FIGS. 3A, 3B and 3C of the accompanying drawings, in accordance with the movement of the objective lens 5 in the direction of the optic axis relative to the object, and forms the image of the light source 1. FIG. 3A shows a case where the objective lens 5 is in its front focus state, FIG. 3B shows a case where the objective lens 5 is in its in-focus state, and FIG. 3C shows a case where the objective lens 5 is in its rear focus state. As is apparent from FIG. 3, the light from the light source 1 reflected by the object has its position of incidence onto the light-receiving element 7 varied in accordance with the focus state of the objective lens 5 and therefore, if the boundary 7C between the first light-receiving area 7A and the second light-receiving area 7B of the light-receiving element 7 is disposed at the converged position of the light during the in-focus of the objective lens 5, detection of the focus state of the objective lens 5 will become possible in accordance with the difference between the outputs of the areas 7A and 7B.
Now, in the device of this type, to avoid the influence of the extraneous light, the light-receiving surface of the light-receiving element 7 is formed in the form of a slit and therefore, the distribution of the reflected light to the light-receiving surface of the light-receiving element 7 in each focus state of the objective lens 5 is such as shown in FIG. 4A of the accompanying drawings. That is, in FIG. 4A, S1 indicates the distribution of the reflected light relative to the light-receiving surface when the objective lens 5 is in its rear focus state, S2 indicates the distribution of the reflected light relative to the light-receiving surface when the objective lens 5 is in its in-focus state, and S3 indicates the distribution of the reflected light relative to the light-receiving surface when the objective lens 5 is in its front focus state. As is apparent from FIG. 4A, during non-in-focus, the distribution of the reflected light is large as indicated by S1 or S3 and most of the reflected light deviates from the light-receiving surface of the light-receiving element 7 and therefore, the light-receiving efficiency of the light-receiving element 7 is remarkably reduced.
Accordingly, in such device according to the prior art, the signal (a-b) indicative of the difference between the outputs of the areas 7A and 7B and the signal (a+b) indicative of the sum of the outputs of the areas 7A and 7B are such as shown in FIGS. 5A and 5B, respectively, of the accompanying drawings, and the output during non-in-focus is low and the detectable distance is limited. In FIGS. 5A and 5B, X indicates the amount of movement of the objective lens 5 and J indicates the in-focus position.